Screw compressor unit

Abstract

In a capacity-adjustable screw compressor unit, a thrust force caused by a reaction force of gas can be appropriately cancelled by a balance piston. A screw compressor unit of the present invention includes a compressor, having a pair of male and female screw rotors meshing with each other contained in a casing, for compressing sucked gas and discharging it, a balance piston for pressing a rotor shaft acting as a rotating shaft of at least one of the screw rotors in an axial direction by a fluid pressure, a slide valve for adjusting capacity of the compressor, and a balance piston control device for adjusting pressure of a fluid applied on the balance piston according to the capacity of the compressor calculated from a position of the slide valve.

What is claimed is: 
     
       1. A compressor unit, comprising:
 a casing; 
 a pair of male and female screw rotors contained in the casing and meshing with each other; 
 a compressor having the casing and the screw rotors and that compresses sucked gas and discharges it through a discharge passage; 
 a balance piston that presses a rotor shaft acting as a rotating shaft of at least one of the screw rotors in an axial direction by a fluid pressure; 
 a slide valve positioned in the casing between the pair of male and female screw rotors and the discharge passage so that a volume of compression space in communication with the discharge passage depends on a position of the slide valve, the slide valve driven by a fluid cylinder; and 
 a balance piston control device, the balance piston control device including
 a positioner that detects a position of a piston of the fluid cylinder that drives the slide valve, 
 a valve adjuster that adjusts an opening of a control valve based on a pressure setting value to thereby adjust pressure of a fluid applied on the balance piston to coincide with the pressure setting value, and 
 a control device that sets the pressure setting value of the valve adjuster based on a detected value of the positioner. 
 
 
     
     
       2. The compressor unit according to  claim 1 , further comprising:
 a suction pressure detector that detects pressure of gas sucked by the compressor, 
 wherein the balance piston control device adjusts the pressure of the fluid applied on the balance piston by taking account of a detected value of the suction pressure detector. 
 
     
     
       3. The compressor unit according to  claim 1 , further comprising:
 a discharge pressure detector that detects pressure of gas discharged by the compressor, 
 wherein the balance piston control device adjusts the pressure of the fluid applied on the balance piston by taking account of a detected value of the discharge pressure detector. 
 
     
     
       4. The compressor unit according to  claim 1 , further comprising:
 a suction pressure detector that detects pressure of gas sucked by the compressor, and 
 a discharge pressure detector that detects pressure of gas discharged by the compressor, 
 wherein the balance piston control device adjusts the pressure of the fluid applied on the balance piston by taking account of detected values of the suction pressure detector and the discharge pressure detector. 
 
     
     
       5. The compressor unit according to  claim 1 , wherein the valve adjuster comprises means for continuously varying the opening of the control valve. 
     
     
       6. The compressor unit according to  claim 1 , wherein the valve adjuster is a PID controller.